Conventional clothes irons include a handle and a heated sole plate. The sole plate is pressed against fabric to remove wrinkles. The iron may also have a heel upon which the iron can rest when it is not in use. The heel allows the iron to sit in a vertical position where the heated sole plate is not in contact with the fabric or an ironing board.
The sole plate of a typical iron usually becomes very hot. Thus, there is a danger that leaving an iron unattended can result in fire or serious damage to the clothing or ironing board. The danger is greatest when the user leaves the iron on and unattended for a long period of time.
To reduce this risk, some irons have automatic shut off devices. The automatic shut off device disconnects the power supplied to a sole plate heater when the iron has not been in use for a prescribed period of time.
Some automatic shut off devices use two separate timing periods to determine when the power should be removed from the sole plate heater. A first timing period is selected for the condition when the iron is left in the vertical position where it is resting on its heel with the sole plate perpendicular to the ironing board. A second timing period is selected for the condition when the iron is left unattended in the horizontal position where it is resting on its sole plate. The first timing period is typically a relatively long time, perhaps ten to fifteen minutes. Since the danger is greater when the sole plate is left in the horizontal position on the clothing or ironing board, the second timing period is typically much shorter, usually around thirty to sixty seconds.
Various arrangements have been devised to determine when the iron is in use and when each timing period should be actuated. For example, some irons use motion sensors or accelerometers. When the iron is moved by the user, the motion sensor, typically a mercury switch, repeatedly resets the automatic shut off timer so that power will not be removed from the sole plate heater when the user is operating the iron. A disadvantage of this type of iron is that it automatically shuts off when held motionless by the user. Such an iron also may not function properly on a non-level surface. Also, chemicals harmful to the environment, such as mercury, are used in many of these motion sensors.
Magnetic switches have also been used to detect when the iron is in use. An example of a magnetic switch is described in U.S. Pat. No. 4,803,342 (Steers). Steers describes the use of a swinging magnet to control timers. Steers has the disadvantage of not providing for an automatic shut off of the iron in the event it is left on its heel for an extended period. Another disadvantage of this type of iron is that the magnet may become demagnetized, thereby preventing the device from operating properly.
Photosensors have also been used to determine if an iron is in use. Typically, a photosensor is placed at a position on the handle of the iron that is covered by the user's hand when the iron is in use, thereby preventing light from reaching the photosensor. A disadvantage of this type of iron is that the iron will remain on if anything blocks the light.
Finally, proximity sensors have been used to determine the user's presence near the iron. A proximity sensor detects the user's presence by sensing the electrical coupling between a user's body and an electrically radiating unit within the iron. Such an iron is shown in U.S. Pat. No. 5,380,983 ((Cavada). Cavada discloses an antenna located in the handle of the iron that serves to conduct a current in conjunction with the user's hand. Cavada's antenna lines the entire inner surface of the iron handle. The current formed by this antenna is then used to control timers consisting of series connected flip-flops. When the user's hand is in proximity to or grasping the handle, a displacement current is formed which resets the flip-flops. The output of the circuit is an amplified square wave signal when the hand is touching the handle of the iron. This square wave signal is applied to the reset pins of the plurality of flip-flops, which prevents them from counting. When the hand is removed, the current stops flowing and the flip-flops begin to count. One disadvantage of Cavada's proximity sensor is that it requires the entire handle to be lined with metal. Another disadvantage is that the use of square wave signals requires substantial circuitry.